The use of high intensity focused ultrasound (HIFU) for cancer therapy has several major advantages over other, more established treatment modalities: it is cheap, non-invasive, and has minimal side-effects. However, widespread acceptance of HIFU is hindered by the lack of a reliable real-time monitoring system.
Above a certain pressure threshold, high-amplitude acoustic waves propagating through tissue can spontaneously nucleate and excite small, micron-sized bubbles, a phenomenon known as acoustic cavitation. The cavitating bubbles re-emit part of the incident ultrasound over a range of frequencies that are different to the HIFU excitation frequency, which is useful for two reasons. Firstly, emissions that have a higher frequency content than the original HIFU source will be absorbed more readily by surrounding tissue, which means that cavitation can greatly enhance heat deposition [Coussios C C, Farny C H, Haar G T, Roy R A. “Role of acoustic cavitation in the delivery and monitoring of cancer treatment by high-intensity focused ultrasound (HIFU)”, International Journal of Hyperthermia vol. 23, pp 105-120, 2007]. Secondly, the broadband acoustic emissions that are associated with this enhanced heating can serve as an indicator of treatment.
Cavitation during HIFU exposure has previously been monitored in either of two ways. One option is to use high-frequency broadband transducers to act as passive cavitation detectors (PCDs) that record the acoustic emissions from cavitating bubbles [C. H. Farny, R. G. Holt, R. A. Roy, “Monitoring the development of HIFU-induced cavitation activity,” AIP Conf. Proc., vol. 829, pp. 348-352, 2006]. PCDs have a fixed focus, however, thereby providing information for a fixed region only. It should be noted that there is currently no cavitation monitoring system that has been adopted in clinical practice. Alternatively, hyperechogenic regions in B-mode ultrasound images can enable detection and localization of bubble activity using time-of-flight information [S. Vaezy, et al., “Real-time visualization of high-intensity focused ultrasound treatment using ultrasound imaging”, Ultrasound Med. Biol., vol. 27, pp. 33-42, 2001]. However, B-mode images can only be taken while the HIFU is off to avoid interference from the therapeutic ultrasound signal and will thus only enable detection of cavities that subsist after HIFU excitation has ceased. B-mode monitoring is therefore less sensitive than PCD monitoring, and has been previously shown to be detecting boiling bubbles that are indicative of overtreatment, rather than inertially cavitating bubbles that are indicative of enhanced heat deposition [B. A. Rabkin, V. Zderic, S. Vaezy, “Hyperecho in ultrasound images of HIFU therapy: involvement of cavitation,” Ultrasound Med. Biol., vol. 31, pp. 947-956, 2005].